1. Field of the Invention
The present invention relates to a method and apparatus for laminating multilayer printed circuit boards having both rigid and flexible portions and, more particularly, to the use of a high pressure vessel into which a gas may be introduced and heated for laminating multilayer printed circuit boards with both rigid and flexible portions.
2. Description of the Prior Art
Multilayer printed circuit boards comprise a number of circuit layers separated by insulation layers which are laminated together to form a solid board. Each circuit layer consists of an insulation layer on which copper circuit traces appear on one or both sides thereof. The insulation layers are typically thin sheets of fiberglass or film dielectric and such insulating sheets are placed between the circuit layers.
In the past, such multilayer printed circuit boards have been laminated together by being placed between a pair of steel plates within a hydraulic press. The press is heated to a predetermined temperature and a pressure is applied for a predetermined time. At times, the multilayer assembly is placed within an air-tight, sealed chamber which is connected to a vacuum pump whereby air bubbles and gases produced during the heating of the materials may be removed.
As technology advances, this method has become unsatisfactory in many situations, for at least two reasons. In the past, multilayer printed circuit boards have either been rigid or flexible. In a rigid circuit board, each circuit layer consists of an insulation layer made from a rigid material. In a flexible circuit board, each circuit layer consists of an insulation layer made from a flexible sheet of material. Recently, a new requirement has developed for circuit boards that have both rigid portions and flexible portions. The lamination of such boards presents a number of difficult technical problems relating to thermal expansion and the even application of pressure during the lamination process. That is, the portions of the multilayer circuit board which are rigid have different coefficients of thermal expansion than the portions thereof which are flexible. Furthermore, the portions of the circuit board which are flexible are typically much thinner than the portions that are rigid so that elaborate inserts must be developed so that the press applies a uniform pressure to the entire circuit board during the lamination process.
The other problem relates to the spacing between the copper circuit traces on the individual circuit layers. That is, the circuit traces are becoming finer and more closely spaced and it is necessary that the insulation material flow uniformly into the spaces between all circuit traces during the lamination process. This is because the function of the insulating layer is not only to insulate the circuit traces on one layer of a mutlilayer board from the facing traces on an adjacent layer of the multilayer board, but also to insulate the individual traces on each multilayer board from each other. The steel plates of conventional hydraulic presses, being rigid, will not conform to the irregularities in the multiple layers and are simply not capable of applying a uniform pressure which will cause the insulation material to flow uniformly into all of the spaces between the circuit traces. Therefore, air becomes entrapped within the layers. This air creates spaces between circuit traces which are not filled with insulation material, resulting in possible shorts and rendering the multilayer printed circuit board subject to possible failure during future operation.
It is known in the prior art to use air pressure to laminate two parts together. The two parts to be laminated together are typically placed in a vacuum bag within a vessel into which air pressure is introduced. The air in the vessel is heated to a predetermined temperature and a pressure is applied for a predetermined time. However, while lamination of two parts using air pressure has been known, it has never been thought possible heretofore that such a technique could be used to perform the intricate lamination required when laminating a large number of complex layers of a multilayer printed circuit board.